I had something like this happen a few years ago when my grandmother was still alive. Nana lived with her daughter in Manchester, U.K. The two ladies sent my daughters Christmas gifts one year. Both packages looked identical - they were both from Manchester and wrapped in brown paper. The kids unwrapped them so quickly, I didn't get a chance to see which card fell out of which package, so I didn't know which lady had sent what. All I knew was that one of them had sent sweaters, and the other pyjamas. So, I took a chance and wrote letters thanking Nana for the sweaters and Aunt for the pyjamas.
You know where this is going, right?
I got a VERY frosty letter back from Nana that basically said "AUNT sent the sweaters, not me" and made me feel like an idiot. Worse, my mother took Nana's side (which was very out-of-character; they didn't get along at all), saying "Why didn't you keep track?" I said "Have you ever seen a five-year-old and a three-year-old unwrap presents? They don't do it slowly or carefully!"